This invention pertains generally to tracking radars, and more particularly to phase-comparison monopulse radars.
It has been recognized for many years that the presence of two targets, or signal sources, within the sum beam of a phase-comparison monopulse radar may reduce the accuracy of tracking by such a radar. Degradation is suffered because signals from the two targets, or signal sources, may combine to form a simple composite signal that, when processed, produces monopulse sum and difference signals corresponding to a point in space different from either of the points occupied by the two targets, or signal sources.
The actual tracking point of a monopulse radar illuminating two closely spaced targets, or signal sources, depends upon the relative amplitudes of the signals received from the two targets, or signal sources. Therefore, if the two targets have complex shapes (as aircraft), or if two signal sources are Gaussian jammers, the parameters of the single composite signal will change with time in a random manner. As a result, then, the tracking point will also vary randomly.
To reduce the deleterious effects of two targets, or signals sources, in the sum beam of a monopulse radar, conventional measures may be taken. For example, the sum beam may be made as narrow as possible and the bandwidth of the receiver may be made as narrow as possible. However, even after such measures are taken, there remain many important situations in which significant tracking errors occur because of the presence of two targets, or signal sources, in the sum beam of a monopulse radar.
There have been some attempts made to process monopulse signals to determine the presence of two targets, or signal sources, in the sum beam and then to correct for the accompanying boresight angle error. For example, Heffner et al (U.S. Pat. No. 4,136,343) teach that the existence of high level (relative to the amplitude of the signal in the sum channel) signals in the elevation and azimuth difference channels is indicative of two targets, or signal sources, in the sum beam of a monopulse tracking radar used as the sensor in a guidance system for a guided missile. By appropriately processing the monopulse sum and difference signals, a guidance command signal may be derived to force the boresight line of the monopulse antenna toward one of the targets, or signal sources. Ultimately, then, only the selected target, or signal source, will remain in the sum beam to provide the requisite tracking signals. Although a simple and reliable way in which to determine whether or not two targets are present in the sum beam of a monopulse radar is taught in the cited patent to Hefner et al, there is not suggestion that the boresight error of each one of two targets should, or may, be measured. Therefore, the system disclosed by Heffner et al would be useful only in tactical situations in which selection of the particular one of the targets, or signal sources, on which a missile is to home is of no concern. Further, it will be noted that, until there is a single target in the sum beam, the measurements required by the system disclosed by Hefner et al must be made continuously.
Leuenberger et al (U.S. Pat. No. 4,084,160) and Schenkel et al (U.S. Pat. No. 4,219,816) teach that angle error components caused by two targets in the sum beam of a monopulse radar may be determined by processing, in addition to the conventional monopulse signals, a so-called "cross-term" signal. With the angle error components determined, an iterative procedure may then be followed to obtain an estimate of the boresight error angle of each of the two targets. Even though the method of either of the just-cited patents may be useful in some tactical situations (specifically in the situation when the radar is being used as a search radar and there is sufficient time to carry out the required iterative procedure) there are situations in which sufficient time is not available to follow the disclosed method in either of the references being discussed. For example, when a radar-guided missile is closing on two closely spaced targets, an iterative procedure to resolve the signals from the two may not be practical.